The present invention relates to the formation and structure of hermetically sealed metal and dielectric thin film optical structures, and in general to structures which include environmentally sensitive thin film coatings of material such as silver and to applications such as metal and dielectric optical bandpass filters.
Metal and dielectric thin film optical coatings are employed in numerous bandpass filter applications, needing high attenuation as well as good transmission and narrow bandwidths. A typical application is medical instruments, such as glucometers, that use the filters to measure the optical properties of a sample at a discrete wavelength.
Unfortunately, unprotected thin films of most metals, in particular silver, will decompose physically and/or suffer deterioration or loss of the desired spectral characteristics, due to humidity-induced tarnishing or oxidation.
In the past, most efforts to hermetically seal thin films have centered on depositing a durable thin film overcoat, epoxying covers on the films, or mechanically sealing the films using organic barriers. These solutions at best provide short term environmental barriers because of the high porosity of glues and organic materials and the short humidity path length of the thin overcoat layers.
To our knowledge, previously solder sealed encapsulation techniques have not been applied to metal-dielectric thin film technology because the films are subject to physical and optical deterioration at the temperatures used by low temperature solders and are attacked by common solder and solder flux materials.